The present invention relates to a method of cyclic desorption of adsorption media cyclically loaded with adsorbates.
Because of the requirements of the environmental protection with respect to retaining the air and water clean, it is important for the future to make exhaust gas or water streams having low concentration of for example solvent or solvent vapors or other gaseous or liquid deleterious substances, such as nitrogen oxides, phenols, and other adsorbates, free from these admixtures. The same is true with respect to removal of hydrocarbons from natural gases, for example casing-head gas or oil gas, and also for cleaning technical gases, such as for example a synthetic gas.
The recovery or regeneration of these admixtures (adsorbates) is performed in cleaning devices suitable for alternating adsorption and desorption on activated carbon with one or several respectively connected reactors. The most consuming step in this process is the desorption of the adsorbate by a vapor (desorption vapor), as a rule water vapor or nitrogen.
Arrangements for regeneration of such adsorbates, for example of solvents with activated carbon arranged in a firm bed, are composed of at least two reactors which make possible removal of the solvent in alternative order between adsorption and desorption uninterruptedly from the gas phase, the reactors being alternately turned on for the adsorption and the desorption. The dimensions of the reactors must be selected as a rule in accordance with the requirements made to the adsorption, wherein the volume stream of the gas phase to be cleaned determines the cross section of the reactor. Since the volume stream of the desorption vapor utilized for desorption is considerably smaller than the volume stream of the gas phase to be treated, the vapor speed during the desorption in the activated carbon bed amounts only to several cm/s.
For drying and cooling of the adsorption medium (in the case of adsorption from the gaseous phase) which as a rule follows the desorption, the moisture repellancy of the utilized adsorption medium determines the physical properties of the adsorbate and the requirements of the individual case. As a rule, the drying and cooling of the desorbed adsorption medium is performed by fresh air which is guided by a blower via a heat exchanger into the cycle. Moreover, it is possible, in the event of air cooling, to exhaust air after exiting from the reactor to be cooled into the atmosphere. Also an inert gas can be utilized, instead of air, as a cooling medium.
In the time sequence in the arrangement with drying and cooling cycle, the adsorption time and the regeneration time are comparable with one another, wherein the regeneration time embraces the time for desorption, drying and cooling.
The process of the conventional desorption of the activated carbon, for example with water vapor, starts in a substantially slow yield (in addition to possible overflowing water vapor) from condensates which are composed in the beginning from the desorbed adsorbate, and later from water. Then only a mixture of adsorbate-vapor and water vapor exits from the reactor, wherein in the middle of the conventional desorption time a ratio of adsorbate-vapor to water vapor is adjusted to 1:1 and in the second half of the desorption time to a ratio of approximately 1:10. This means that by this time point 10 kg of water vapor is needed for desorption of 1 kg of adsorbate. Thereby the vapor consumption is so high that the effective desorption no longer takes place.
The known desorption methods possess considerable disadvantages. The alternating utilization of the reactor or reactors for adsorbing and desorbing leads, because of the dimensioning of the reactor size upon the requirements made to the adsorption, in the event of desorption to very small vapor speed. A uniform action upon the activated carbon bed with water vapor is in question, especially when relatively great reactor sections are treated with small layer heights in comparison with the former. Moreover, the requirement to utilize economically the vapor quantity leads to limiting the desorption to an increased rest loading and thereby to reduced loading ability in the next cycle and, as a result, to a reduced utilization of the adsorption property of the activated carbon accommodated in the reactor.